1. Field of the Invention
The present invention relates generally to electrical switchgear, and more particularly to arc resistant switchgear, and even more particularly to arc resistant switchgear that features dedicated vertical plenums.
2. Description of the Related Art
This invention is directed to the providing of arc resistant switchgear featuring a dedicated vertical plenum for each vertical switchgear cubicle. Arc resistant switchgear is electrical equipment designed to withstand the effects of an internal arcing fault. The occurrence of arcing inside electrical switchgear has many undesirable results. The arc energy can cause a sudden pressure increase inside the enclosure resulting in severe mechanical and thermal stress on the equipment, particularly the door.
The arc itself can be caused by a bad breaker, the presence inside the switchgear of a small animal, bad insulation, or even lightning. As arcing begins, the gases inside the switchgear ionize. This ionization reduces the ability of the air inside the switchgear to provide its previous insulative capabilities, and in fact the ions generated by the degradation of the air molecules inside the switchgear causes the air to more readily conduct electricity. Consequently, any given arc tries to get more paths to ground. Further, the existence of an arc results in the temperature of the air in the enclosure rising quickly due to the high level of heat energy, since the temperature of the arc alone is can be 10,000-20,000° F.
As the gases expand rapidly, there is a rapid build-up in pressure. The combination of the rapid build-up in heat and the rapid build-up in pressure can result in an explosion with a shockwave that can measure Mach 2. If an individual is nearby the unit where the arcing occurs, they can be seriously injured, if not killed. There have been instances where the hot gases alone have been responsible for igniting the clothing of nearby workers, since the exhaust temperature alone can reach 2000° F.
At the very least, in an explosion due to arcing occurring inside switchgear, substantial material damage can occur, resulting in significant economic loss due to the interruption of energy distribution not to mention the destruction of the switchgear itself. This material damage can be to the door itself, or it can also involve the destruction of parts from within the switchgear unit, or burn through of the metal cubicles housing the switchgear unit. While one might think that all of the aforementioned process takes a considerable time to fully develop, in actuality the compression of the gas within the enclosed space due to the increase in pressure due to arcing, and the subsequent expansion of the gas as described above can all occur between 5-15 milliseconds.
Prior art electrical switchgear typically utilizes two main orientations of cubicles. In the one type of orientation, a vertical section of electrical components, typically breakers, separate two pairs of compartments, each pair of compartments having an upper cubicle and a lower cubicle, with the front pair of cubicles having doors facing the front of the switchgear unit, and with the rear pair of cubicles having doors facing the rear of the switchgear unit. In the event of an explosion, the exhaust theoretically exits from the affected cubicles through a vent or panel located in the top ceiling of each cubicle needing to be vented, with the exhaust passing into the room where the switchgear is located. In actuality, the door is typically forced open due to the force of the explosion and/or the metal housing of the cubicle can burn through.
In another type of prior art orientation of the cubicles, an internal horizontal plenum or passageway runs through a number of directly adjacent vertical pairs of upper and lower cubicles, with the upper cubicles often being vented by the type of vent discussed in the preceding paragraph. In addition, the cubicles underneath the internal horizontal plenum each have a vent located in their top ceiling, such that if an explosion occurs, the exhaust is forced upwardly into the internal horizontal plenum, and then ultimately discharged over the top of the switchgear typically into the air of the room where the switchgear is located. In switchgear having vents only in the ceiling, or in a combination of an internal horizontal plenum and ceiling vents, typically at least an 8′ vertical clearance between the top of the cubicle and the ceiling of the room where the switchgear is located must be maintained due to the force of any blast exhaust.
While it is apparent that the problem associated with arcing in electric switchgear has been recognized, it is also readily apparent that the need continues to exist for arc resistant switchgear. Attempts have focused on lessening the chance of an arc occurring, but when one occurs, the only way to minimize it is by quickly providing for the safe exhaust of the hot gases. Existing methods have not adequately solved the problem.